Canada warbler

Although not yet listed as an at-risk species in Ontario, the country’s namesake warbler is one of the continent’s fastest dwindling migratory songbirds

by Tim Tiner

It is a painful irony that one of the newest additions to Canadas list of species at risk is the countrys namesake warbler. A prolonged slide in population has made the much-loved, loquacious Canada warbler one of the continents fastest dwindling neotropical migrants. Last April, the federal Committee on the Status of Endangered Wildlife in Canada designated it as threatened. Yet a paucity of research on the imperilled necklaced warbler, which nests throughout most of Ontario into the northern boreal forest, leaves the primary cause of its decline largely unclear.

Canada warblers are so little studied partly because they have never been abundant. They are limited to a fairly specific niche, inhabiting dense,log-strewn understoreys in damp, mossy woods, usually near forest edges, swamps and bogs and often in ravines or on steep slopes leading to waterside thickets of alders and willows. Their nests, well hidden in moss hummocks, rotting stumps and ruts created by upended trees, are seldom found, obscured by thick fern beds and tree roots.

Foraging mostly within five metres of the ground, the hyperactive, lemon-breasted birds flit constantly about, snapping flying insects from the air like a flycatcher and gleaning spiders from shrubs and saplings. Though these birds are difficult to see, blending with the leaves in their tangled surroundings, blue-grey-backed males sing throughout the day in loud, distinctive but highly varied phrases of five to 15 explosive, sputtering notes. Individual birds have repertoires of up to 11 different songs.

Those songs, however, are ringing out less and less in forests across Ontario and the rest of the country. Canada warbler tallies in the annual Canadian Breeding Bird Survey have dropped by 85 percent over the past four decades, and the decline has accelerated in recent years. Findings in the northeastern United States are similar.

The Ontario government does not yet list the Canada warbler as a species at risk in the province. Based on data from the second edition of the Atlas of the Breeding Birds of Ontario, published last year, the province is estimated to have a population of 450,000 breeding pairs, with the biggest concentrations along the southern Canadian Shield and in northeastern Ontario. In the Carolinian region, where the warblers were already rare, probability of observation fell by 36 percent in the 20 years between Ontarios first and second bird atlas projects. The loss of swamps and damp forests is believed to have a lot to do with the species decline in heavily settled parts of the province and the northeastern United States.

Losses, however, are more confounding in the warbler’s southern Canadian Shield stronghold, where sightings declined by 10 percent in the years between the two bird atlas projects. Definitive explanations are elusive, though some researchers speculate that dense young forests that grew up on abandoned marginal farmlands early in the past century have thinned and matured, becoming unsuitable for the warblers.

Given that 85 percent of the Canada warbler’s global breeding population is in Canada, the species seems aptly named. Yet the bird is really a tropical sojourner, making only the briefest of nursery visits. It is one of the last warblers to arrive in spring, reaching Ontario in late May and early June. It takes only about three weeks from the time females lay their last egg until their broods, though still unfledged, leave their nests. Parents may feed their young for another two or three weeks, but many head south again by mid-August after less than three months in the north.

Canada warblers spend much more of their lives in humid mountain forests on the east side of the Andes, from Colombia to central Peru. Habitat loss in their southern range, however, has been far more severe than in the north; about 90 percent of the tree cover in the northern Andes has been cleared since the 1970s. To some extent, the warblers find alternative havens, such as coffee plantations.

But on their breeding grounds, Canada warblers are very susceptible to disturbance. Research shows they successfully raise a relatively low number of offspring. Some studies suggest they are more sensitive to forest fragmentation than most other woodland birds. They are also more scarce in forests that have been thinned, and the species could be affected by management practices that remove understorey shrubs. Canada warblers are also found less often in heavily browsed forests with high deer populations.

One recent study links this warbler’s decline with the spruce budworm, which defoliates and occasionally kills primarily balsam fir over vast areas during outbreaks that recur about every 30 to 35 years. Canada warblers thrive when budworm densities become so high that the caterpillars begin raining down from the tops of mature firs into the hungry mouths of the subcanopy habitués. The statistical study points to steadily falling warbler numbers since the area of budworm defoliation in Canada reached 52 million hectares in 1975; in 2005, by comparison, the defoliated area was 700,000 hectares. “The population growth rate of both species is highly correlated,” says one of the study’s co-authors, Kandyd Szuba, a biologist with Domtar Inc. “The data seems to suggest that the decline of Canada warblers is a natural phenomenon.”

Steve Holmes, a research scientist with the Canadian Forest Service, however, is unconvinced. He notes that while Canada warblers have decreased steadily in Ontario over the past 40 years, budworm infestations peaked around 1980 on the east side of the province, in the mid-1980s in the northwest and in the early 1990s in central Ontario. “Whether there is a cause and effect, I would question. I don’t think it’s been proven,” says Holmes.

All agree that considerably more research, especially in the field, is needed on many aspects of the Canada warbler puzzle. At present, however, no such studies are underway in Ontario. Under the federal Species at Risk Act, Environment Canada must complete a recovery plan for the songbird. Ontario’s new Endangered Species Act also requires the province’s Ministry of Natural Resources to assess and classify any new species the federal government has listed, which could lead to greater protection for the Canada warbler.

FIND OUT MORE ABOUT CANADA WARBLERS

Committee on the Status of Endangered Wildlife in Canada (COSEWIC) www.sararegistry.gc.ca/document/dspText_e.cfm?ocid=6282
COSEWIC’s April 2008 Assessment and Status Report on the Canada Warbler, including in-depth information on the natural history and threats to the species, can be found in its entirety at this site.

A Field Guide to Warblers of North America Written by Jon Dunn and Kimball Garrett and published in 1997, this book in the Peterson Field Guides series provides detailed species descriptions and entries on habitat, migration, behaviour and conservation.

Tim Tiner is the author of several nature guide books and a longtime contributor to ON Nature.